The dispenser cathode may be divided into two types, the impregnation type and the cavity reservoir type. The impregnation type dispenser cathode includes an electron releasing substance, such as an alkaline rare earth compound, in the cavities of a porous metal substrate, while, in the cavity reservoir type cathode, the porous metal substrate and electron releasing substance fill a cup in a stratified form.
The electron releasing material used in the dispenser cathodes is mainly a compound prepared by mixing MgO, SrO, Sc.sub.2 O.sub.3 and rare earth metallic oxide with, as the chief ingredient, BaO or Ba. Ca. aluminate obtained by sintering BaO, CaO and Al.sub.2 O.sub.3.
The materials preferable for the porous metal substrate are powders of W, Mo, Ir, Re, Os, Ru or powders of alloys thereof which have high melting point, anti-ionization and antishock characteristics. The manufacturing process for the porous metal substrate of the dispenser cathode as explained above includes a step of forming it by means of a press jig, a step of sintering it under an atmosphere of hydrogen gas or a vacuum, and a step of impregnating into it an electron releasing material.
The dispenser cathode as described above is capable of a high thermal electron releasing state for a long time, is applicable to electron tubes for to diversified purposes, and the research for its improvements and being currently carried out very intensively.
However, such a conventional dispenser cathode is operable at a temperature range of 1050.degree.-1200.degree. C., and its operability only at the high temperature imposes various limitations on designing the components. That is, a heater having a thermal capacity larger than that of the general oxide cathodes has to be provided, and the nearby components such as cup and sleeve have to be made of a heat resistant metal. Further, there are the problems that the electron releasing material within the cavities of the porous metal substrate can be evaporated upon being heated to a high temperature during operation, shortening its life expectancy and that the evaporated electron releasing material can adhere to nearby components, degrading the performance of the electron tube
An improved impregnation type cathode intended to lower its operating temperature has been proposed, and this dispenser cathode is manufactured by coating a thin layer of Os,Os alloy or Ir on the thermal electron releasing face of the porous metal substrate. In this cathode, while the operating temperature is lowered by about 150.degree. C., there is the problem that the coated layer is weak against ion impacts, and thus the life expectancy of the cathode is shortened. (Refer to U.S. Pat. No. 4,417,173).
There has been another proposal for the impregnation type cathode in which Sc ingredient is added to the porous metal substrate containing W as the chief ingredient or the surface of the porous metal layer substrate is coated with a W and Sc.sub.2 O.sub.3 layer, but this dispenser cathode has not been able to attain to a satisfactory level yet. (Refer to U.S. Pat. No. 4,783,613)
There has been a proposal for the cavity reservoir type cathode as shown in FIG. 1, in which a first pellet 1 made of a mixture of barium calcium aluminate and W is inserted into a reservoir 2 secured at the upper position of a sleeve 4, and a second pellet 3 made of W or alloys thereof seals the sleeve 4, this dispenser cathode being disclosed in U.S. Pat. No. 4,823,044. The operating temperature of this dispenser cathode is 850.degree.-1000.degree. C. which is higher by over 150.degree. C. than the operating temperature (.apprxeq. 750.degree. C.) of the oxide type cathode, and therefore, there remain the limitation of the design and other disadvantages, although there are some differences.